<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    
    <style>
        main div:nth-child(2){
            border: 3px solid silver;
            width: 100px;
            height: 100px;
            /* box-sizing: border-box; */
            float: right;;
            background-color: tomato;
            
        }
        main div:nth-child(1){
            background-color: green;
            width: 100px;
            height: 100px;
            float: left;
            /* clear: both; */
            
        }
        main div:nth-child(1)::after{
            content: '';
            clear: both;
            display: block;
        }
        main{
            background-color: silver;
            overflow: auto;
            height: 100vh;
            border: 12px solid silver;
            background-clip: content-box;
            padding: 10px;
        }

        main div:nth-child(3){
            width: 100px;
            height: 100px;
            background-color: blueviolet;
            /* float: left; */
            /* clear: both; */
            
        }
    </style>
<main>
    <div>第一个</div>
    <div>第二个</div>

    <div>第三个DIV</div>
</main>
    



</body>
</html>